The need for systems that can deliver any drug at a controlled rate of release to an environment of use over a specified period of time is well established.
U.S. Pat. No. 4,814,182 discloses the use of rods or slabs of pre-hydrated and swelled polyethylene oxide hydrogel. The polymer is impregnated with a biologically active agent during the hydration procedure. The hydrated polymer is then dried and partially coated with an impermeable, insoluble material. When placed in an aqueous environment, the polymer swells but does not dissolve or disintegrate. The entrapped active ingredient is released from the polymer by diffusion. The mechanism of release is based on the ability of the soluble drug to diffuse through the rehydrated hydrogel and move into the aqueous environment.
U.S. Pat. No. 4,839,177 discloses the use of hydrogels compressed to defined geometric forms. In this device, the polymer is mixed with biologically active ingredients to form a core which is affixed to a "support platform" made of an insoluble polymeric material. When hydrated, the swellable, gellable hydrogel expands beyond the device and establishes a superstructure from which the active agent is released either by diffusion, if the active agent is soluble, or by erosion, if the active agent is insoluble. The generation and maintenance of the superstructure is vital to the proper operation of this device.
An osmotic dosage form which utilizes a semipermeable wall containing at least one "exit means" which passes through the wall, surrounding a core containing an osmotic agent, a neutral and ionizable hydrogel and an active ingredient is taught in U.S. Pat. No. 4,971,790. The coating of this device is permeable to water from the environment of use. Water moves into the core through the semipermeable membrane. Once inside the device, the water solubilizes the osmotic agent, and hydrates the hydrogels. Pressure builds up inside the device. Ultimately, the solubilized hydrogel, containing the beneficial agent, and other core excipients are pumped out of the core, under pressure, through an exit means and into the environment of use.
The existing technology is limited since diffusion controlled systems are effective only when soluble active agents are dispensed. For osmotically controlled devices, the technology relies upon a wall permeable to the passage of fluid present in the environment of use. Furthermore, these devices require a wall of carefully controlled permeability.
Devices which rely upon the establishment of an extra device superstructure can be altered during in vivo transit, for example, in the gastrointestinal tract. If portions of the superstructure break away, greater surface area is exposed to the environment and unpredictable release of the active agent may result.
The usefulness of the above devices would be increased if a device and method were provided to improve the delivery of drugs independent of their solubility so that diffusion from a swelled polymer or through the superstructure of a polymeric matrix could be avoided. Further utility would result from a methodology which provides a device where the generation of an extra tablet structure could be avoided and the dry ingredients could be contained within a protective coating until released from the device. This would prevent the chance of premature erosion and uncontrolled release of the active agent as well as provide enhanced stability for those active agents that are labile in the fluid of the environment of use.